Destroyer Question: Ortho > Para?

[busupshot83]

Problem #15 in the Destroyer's orgo section has a Cl being added to a benzene with an activating substituent (NH). I understand activating groups direct to the ortho and para positions, but why does it say the ortho is MINOR while the para is MAJOR?

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[poc91nc]

Could be because of steric factors...the Loudon book talks about this in ch16.
Hmmm...the steric interaction could come from the chlorine lone pairs and oxygen on the carbonyl group. Since we are dealing with an amide, the nitrogen is predominantly sp2 and the lone pair is involved in a double bond...if you draw the corresponding resonance structure...the oxygen bears a negative charge (amides are not very reactive relative to other carboxy acid derivatives like esters and acyl chlorides since amides are resonance stabilized). So...unfavorable repulsions between the oxygen and chlorine at the ortho position might play a role in para dominance....that is the only way I can rationalize it.

Lots of other reactions give para dominance...like nitration of bromobenzene...acylation of toluene...etc

 

[jdmsamurai]

um NH is not an amide its an amine...no carbonyl but overall amide is activating

 

[poc91nc]

Ph-NHCOPh is an amide....not aniline...look at the problem.

Aniline is more electron donating than the amide...the amide nitrogen is not as free to donate the lone pair

 

[poc91nc]

Yes...amides are activating due to electron donating...but not as good as aniline...big difference.

 

[busupshot83]

Thanks guys.

 

[cdmOMR]

Ya I think its mostly steric hinderence at the ortho position. You will get both since both have the sigma framework stabilizing effect of added resonance forms, but para is less hindered so I think generally you are going to get more para.